The present invention relates to inhibiting the spontaneous ignition of low rank coals. More particularly, the invention relates to improving a treatment in which the coal is dried while being oxidized and cooled in order to increase its stability.
Numerous treatments have been designed for oxidizing coal to reduce its content of sulphur and ash. For example, U.S. Pat. No. 3,909,211 describes heating coal at 100.degree.-500.degree. F. with gas inclusive of NO.sub.2. U.S. Pat. No. 4,022,588 describes reacting the coal with specified metal oxides then washing or extracting the treated coal. U.S. Pat. Nos. 4,097,244 and 4,015,416 describes treating the coal with an aqueous solution containing an iron complexing agent and a metal oxidant and reacting the so-treated coal with a hydrogen donor. U.S. Pat. No. 4,183,730 describes reacting coal with an aqueous solution of hydrogen peroxide and sodium chloride, then washing the treated coal. U.S. Pat. No. 4,256,464 describes contacting the coal with a liquid organic solvent for nitrogen oxides, then reacting it with gas containing oxygen and nitric oxide. U.S. Pat. No. 4,328,002 describes treating coal with an aqueous oxidizing agent, washing it, reacting it with oxygen to an extent causing an exothermic temperature peak, then washing and drying the treated coal.
Oxidative processes have also been proposed to reduce the molecular weight of the coal and/or render parts of the coal soluble in organic solvents. U.S. Pat. Nos. 2,242,822 and 2,338,634 relate to oxidizing coal, first with air, then with nitric acid, in order to generate hydroxycarboxylic acid groups that cause the non-fusain organic materials to become soluble in organic solvents containing heterocyclic oxygen, with ammonium nitrate or other oxides being used as catalysts in the oxidizing step.
Somewhat milder oxidative processes have been described for inhibiting the spontaneous ignition of low rank coals. For example, U.S. Pat. No. 3,723,079 describes treating coal which has been dried to about 1-10 percent moisture content by means of an air oxidation that provides an oxygen uptake of about 0.5 to 8 percent then rehydrating the coal. U.S. Pat. Nos. 4,396,394 and 4,401,436 describe sequentially drying the coal to near the moisture content desired for the product, oxidizing it by an upflow of hot oxygen-containing gas, then cooling the treated coal to less than about 100.degree. F., or doing the cooling while adding water in order to increase the rate of cooling by the water evaporation.